Gasoline is a well know fuel, generally composed of a mixture of several hydrocarbons including aromatics, olefins, naphthenes and paraffins having different boiling points at atmospheric pressure. The benzene specification in the gasoline is a key parameter for further use of gasoline. The primary sources of benzene in the gasoline are the gasoline blending feedstocks, which include naphtha from fluid catalytic cracker (FCC) units and catalytic reformate products (reformate). While the FCC naphtha is the largest blending stock for gasoline and constitutes up to 50% of the final product, the FCC naphtha itself typically contains about 1% benzene and is therefore not a primary contributor. The reformate product normally contains more than 5% benzene and given that approximately 75% of the benzene that is present in gasoline is derived from reformate. To comply with the regulation for benzene specification in gasoline, many refineries have implemented various techniques and strategies to reduce the levels of benzene in gasoline, which generally contains about 2% to about 3% benzene.
Traditionally, chemical processes are used to convert benzene to other desirable and less objectionable components for reducing benzene content in gasoline blending reformate. The chemical processes are followed by physical separation that separates at least a portion of benzene. Other approaches include alkylation of benzene to yield heavier aromatics whose presence in gasoline was more acceptable. These techniques generally consisted of alkylating benzene with light olefins. Unfortunately, many of the alkylation processes were accompanied by undesirable side reactions and all of these techniques increased the costs to gasoline production significantly. Alkylation techniques are described, for example, in U.S. Pat. No. 3,293,315 to Nixon, U.S. Pat. No. 3,527,823 to Jones U.S. Pat. Nos. 4,140,622 and 4,209,383 both to Herout et al., and U.S. Pat. No. 4,849,569 to Smith. Another known approach of reducing the levels of benzene in reformate was to convert benzene into cyclohexane. However, the process is not selective only to benzene and therefore yields a number of undesired by-products. U.S. Pat. No. 5,294,334 to Kaul et al. and U.S. Pat. No. 5,210,333 to Bellows et al. each disclose processes which selectively adsorb benzene from a gasoline stream and thereafter hydrogenate the benzene into cyclohexane without the need for added desorbents. A drawback of these approaches is that since the cyclohexane remains in the gasoline stream, there is a significant reduction in the grade of the gasoline because the octane rating of cyclohexane is much lower than that of benzene.
The other conventional techniques include pretreatment of reformer feed for removal of benzene precursors from the reformer feed or changing the catalyst and operating pressure in reformer operation to reduce the levels of benzene in the gasoline blending stock. All of these approaches have advantages and disadvantages, typically requiring high equipment and capital costs. There is a need for an improved process and apparatus to reduce the benzene levels in gasoline at reduced equipment and capital costs that can be used in grassroot and revamp applications. Further, in the traditional processes to remove benzene from gasoline, the hydrogen and LPG gases are lost as low value off-gas. Therefore, there is a need for a new process and apparatus to remove benzene from gasoline that enables recovery of hydrogen and LPG gases at reduced capital costs.